Image pickup apparatus having computational gamma correction facility

ABSTRACT

There is provided a novel and improved image pickup apparatus which can be reduced in size and low power consumption because of its reduced circuit scale and in which no degradation of image quality occurs at the switching points between a plurality of functions. Such an image pickup apparatus comprises a non-linear circuit for performing non-linear correction of a digitized video signal, and the non-linear circuit includes a multiplier having an input terminal formed by input bits the number of which is n 1 , n 1 , being selected to meet a relationship of n 3  &gt;n 1 , where n 3  represents the number of input bits of the non-linear circuit. The non-linear circuit also has predetermined input ranges R i  each of which is selected to be R i  ≦2 n1  -1, and is formed by a combination of characteristics of the respective predetermined input ranges R i .

This is a divisional application under 37 CFR 1.62 of prior applicationSer. No. 08/174,124, filed Dec. 23,1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an image pickup apparatusand, more particularly, to an image pickup apparatus capable of varyingits gamma correction characteristic by using digital signal processing.

2. Description of the Related Art

With the recent advancement of electronic technology, image pickupapparatus are being reduced in size and weight. In such a situation,with the advancement of semiconductor technology, high-speedanalog-to-digital converters (hereinafter referred to as "A/Dconverter(s)") and high-speed digital-to-analog converters (hereinafterreferred to as "D/A converter(s)") have been put into practice. It isalso proposed to provide a system for performing digital signalprocessing of digitized video signals by using an A/D converter and aD/A converter. Such a system is generally arranged to perform A/Dconversion of a picked-up image signal, perform signal processing, suchas filter processing, color separation, gamma processing, kneeprocessing and matrix processing, by means of digital processing,perform D/A conversion of the digitally processed signal, and output theobtained analog signal.

In another system, an picked-up image signal is converted from analog todigital (A/D), and digital techniques are used to perform processing,such as clipping, gamma correction, filter processing, addition of async signal, detection of a sync signal and matrix processing, therebygenerating a television digital signal. The television digital signal isconverted from digital to analog (D/A) and the analog video signal isoutputted. There is yet another system in which a television signal isoutputted as a digital video signal.

In such a conventional example, a ROM table is used as a non-linearcircuit, such as a gamma correction circuit or a knee circuit. The ROMtable utilizes a ROM in which predetermined input-output characteristicsare written.

In the image pickup apparatus using the conventionally proposed digitalsignal processing, a read-only memory (ROM) is employed to form, forexample, a gamma circuit. Since the use of this read-only memoryinvolves an increase in the circuit scale of the apparatus, it has beenimpossible to reduce the cost required to integrate all signalprocessing circuits into a single integrated circuit.

Also, the power consumption of ROMs is large, and if a ROM is to beformed as part of an integrated circuit, the ROM will need a large chiparea. For this reason, it is difficult to form a ROM as a highlyintegrated circuit, so that it is difficult to realize an image pickupapparatus of small size and light weight.

In addition, there is the problem that an extremely large ROM is neededfor widening an input dynamic range or realizing a finer characteristiccurve.

As is known, it is occasionally necessary to finely adjust a non-linearcharacteristic curve in accordance with the kind or conditions of asubject to be photographed. For example, if a subject is to bephotographed under low S/N conditions when it is dark, it is desirableto make the non-linear characteristic closer to a straight line since anincrease in noise in a dark portion can be suppressed. If a subject ofhigh contrast is to be photographed, for example, outdoors, it isdesirable to decrease, particularly, the inclination of a high-luminanceportion of the non-linear characteristic since it is possible to reducea gradational deterioration due to the saturation of the high-luminanceportion.

However, in the conventional systems, since an extremely large ROM isneeded, it has been substantially impossible to realize such a fineadjustment.

Further, in the image pickup apparatus employing the conventionallyproposed digital signal processing, since a circuit conventionally usedin analog systems is merely replaced with a circuit for digital systems,the circuit scale is large and a large current consumption occurs. Thislarge circuit scale makes it impossible to integrate all signalprocessing circuits into a single integrated circuit or to reduce therequired cost. Particularly in a conventional type of non-linearcircuit, such as a gamma correction circuit, since an arrangement usinga ROM is adopted, an extremely large circuit scale is needed.

The above-described types of image pickup apparatus have a furtherproblem. If an analog system is replaced with a digital system having avariable characteristic, the circuit scale will increase to animpractical extent. This excessive increase of the circuit scale willalso make it difficult to realize a satisfactory characteristic by meansof a simple system. This leads to the problem that it is impossible toattain an appropriate setting according to the state of photography orthe conditions of a subject to be photographed.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to solve theabove-described problems.

Another object of the present invention is to provide a novel andimproved image pickup apparatus which can be reduced in size and lowpower consumption because of its reduced circuit scale and in which nodegradation of image quality occurs at the switching points betweenindividual functions.

To achieve the above-described objects, according to one aspect of thepresent invention, there is provided an image pickup apparatus whichcomprises a non-linear circuit for performing non-linear correction of adigitized video signal. The non-linear circuit includes a multiplierhaving an input terminal formed by input bits the number of which is n₁,and n₁ is selected to meet a relationship of n₃ >n₁, where n₃ representsthe number of input bits of the non-linear circuit. The non-linearcircuit also has predetermined input ranges R_(i) each of which isselected to be R_(i) ≦2^(n1) -1, and is formed by a combination ofcharacteristics of the respective predetermined input ranges R_(i).

According to another aspect of the present invention, there is providedan image pickup apparatus which comprises a non-linear circuit forperforming non-linear correction of a digitized video signal, thenon-linear circuit including a function circuit capable of varying anon-linear characteristic, and detecting means for detecting a level inan picked-up image signal.

According to another aspect of the present invention, there is providedan image pickup apparatus arranged to convert a picked-up image signalfrom analog to digital and perform signal processing in a digitalmanner. The apparatus comprises a gamma circuit having a gamma region inwhich a gamma characteristic is obtained by adding together a straightline passing through an origin and a first curve and a region other thanthe gamma region in which a predetermined characteristic is obtained byadding together a predetermined value and a second curve.

According to another aspect of the present invention, there is providedan image pickup apparatus which comprises a non-linear correctioncircuit of a type which is arranged to obtain a non-linear correctioncharacteristic by switching of a plurality of functions, and thenon-linear correction characteristic is varied by varying constant termscontained in the respective plurality of functions.

According to another aspect of the present invention, there is providedan image pickup apparatus which comprises an amplifier for amplifying anoutput signal of an image sensor, the amplifier having a gain which isvariable, a first adder for adding a first value to a digital picked-upimage signal obtained by analog to digital conversion of an outputsignal of the amplifier, a non-linear correction circuit for performingnon-linear correction of an output signal of the first adder, a secondadder for adding a second value to an output signal of the non-linearcorrection circuit, a multiplier for multiplying an output signal of thesecond adder by a third value, and a switch for generating a controlsignal S for controlling the gain of the amplifier, the first value, thesecond value and the third value. A characteristic of the non-linearcorrection is varied by varying the gain of the amplifier, the firstvalue, the second value and the third value.

According to one embodiment of the present invention, there is providedan image pickup apparatus which includes a non-linear circuit using amultiplier. The non-linear circuit is formed by a combination of thecharacteristics of the respective predetermined input ranges R_(i) eachof which is selected to meet n₃ >n₁ and R_(i) ≦2^(n1) -1, where n₃represents the number of input bits of the non-linear circuit and n₁represents the number of input bits which form one input side of themultiplier. With this arrangement, since it is possible to realize anon-linear circuit without using a ROM, it is possible to achieve areduced circuit scale, a cost reduction, a reduction in powerconsumption and a reduction in the entire size of the apparatus.

According to another embodiment of the present invention, a non-linearcircuit used in an image pickup apparatus varies a non-linearcharacteristic by varying constants contained in individual functioncircuits in accordance with a level detected by detecting means.

The above and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof preferred embodiments of the present invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram schematically showing a firstembodiment of an-image pickup apparatus according to the presentinvention;

FIG. 2 is a block diagram showing in detail the gamma circuit shown inFIG. 1;

FIGS. 3(a) and 3(b) are explanatory views showing the operation of thefirst embodiment shown in FIG. 1;

FIG. 4 is a block diagram showing another arrangement example of thegamma circuit shown in FIG. 1;

FIG. 5 is a functional block diagram schematically showing a secondembodiment of the image pickup apparatus according to the presentinvention;

FIG. 6 is a block diagram showing in detail the gamma circuit shown inFIG. 5;

FIGS. 7(a) and 7(b) are explanatory views showing the operation of thesecond embodiment shown in FIG. 5;

FIG. 8 is a flowchart showing the operation of the second embodimentshown in FIG. 5;

FIG. 9 is a block diagram schematically showing a third embodiment ofthe image pickup apparatus according to the present invention;

FIG. 10 is an explanatory view showing the operation of the thirdembodiment shown in FIG. 9;

FIG. 11 is a detailed block diagram of the gamma circuit shown in FIG.9;

FIGS. 12(a), 12(b) and 12(c) are explanatory views showing the operationof the third embodiment shown in FIG. 11;

FIG. 13 is a detailed block diagram of the function circuit shown inFIG. 11;

FIG. 14 is an explanatory view showing the operation of the functioncircuit shown in FIG. 13;

FIG. 15 is a block diagram schematically showing a fourth embodiment ofthe image pickup apparatus according to the present invention;

FIG. 16 is a detailed block diagram showing one example of the gammacorrection circuit used in the fourth embodiment shown in FIG. 15;

FIGS. 17(a), 17(b) and 17(c) are explanatory views showing the operationof the gamma circuit shown in FIG. 16;

FIG. 18 is a flowchart showing the operation of gamma control executedin the fourth embodiment;

FIG. 19 is a detailed block diagram showing another example of the gammacorrection circuit;

FIG. 20 is a block diagram schematically showing a fifth embodiment ofthe image pickup apparatus according to the present invention; and

FIGS. 21(a) and 21(b) are explanatory views showing the operation of thefifth embodiment shown in FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 is a functional block diagram schematically showing a firstembodiment of an image pickup apparatus according to the presentinvention. The arrangement shown in FIG. 1 includes a CCD 1 which servesas an image sensor and has a color separating filter, a sample-and-holdcircuit (S/H) 2 for converting the output signal of the CCD 1 into acontinuous signal, an A/D converter (ADC) 3 for converting the outputsignal of the sample-and-hold circuit 2 into a digital signal, and alow-pass filter (LPF) 4 for allowing the passage of only the luminancesignal of a picked-up image signal.

The shown arrangement also includes a gamma circuit 5 which is oneexample of a non-linear circuit, a delay circuit 6 for delaying theluminance signal by a predetermined time period based on a color signalwhich will be described later, a sync signal adder 7, a D/A converter(DAC) 8, and a luminance signal (Y) output terminal 9.

The shown arrangement further includes a color separating circuit 10 forseparating color signals from the picked-up image signal, a multiplier11 for performing white balance adjustment, a coefficient multiplier 12for outputting a coefficient for use in white balance adjustment, agamma circuit 13, a color-difference matrix 14 for formingcolor-difference signals, a modulator 15 for modulating thecolor-difference signals by using a chrominance subcarrier, a burstsignal adder 16, a D/A converter (DAC) 17, and a chrominance signal (C)output terminal 18.

In operation, a subject image (not shown) which has been formed on theCCD 1 by an optical system (not shown) is subjected to color separation,followed by photo-electric conversion, by the CCD 1. The electricalsignal outputted from the CCD 1 is converted into a continuous signal bythe sample-and-hold circuit 2, and the continuous signal is convertedfrom analog to digital by the A/D converter 3. The digitized signal isinputted to the low-pass filter 4 and a luminance signal Y is taken outfrom the digitized signal by the low-pass filter 4. The luminance signalY is gamma-corrected by the gamma circuit 5. Then, after thegamma-corrected luminance signal Y is delayed by a predetermined amountby the delay circuit 6, a sync signal is added to the delayed signal bythe sync signal adder 7. The output signal of the sync signal adder 7 isconverted into an analog signal by the D/A converter 8, and the analogsignal is outputted through the Y output terminal 9 to externalequipment (not shown), such as a television set or a video tape recorder(VTR).

The output signal of the A/D converter 3 is also separated into primarycolor signals R, G and B by the color separating circuit 10. Themultiplier 11 multiplies each of the primary color signals R, G and B bythe coefficient set by the coefficient multiplier 12, thereby performingwhite balance adjustment for adjusting the level of each of the primarycolor signals R, G and B. The output signal of the multiplier 11 isgamma-corrected by the gamma circuit 13, and the output signal of thegamma circuit 13 is formed into color-difference signals by thecolor-difference matrix 14. The modulator 15 quadrature-modulates thecolor-difference signals by using a chrominance subcarrier. Thequadrature-modulated output signal of the modulator 15 is inputted tothe burst signal adder 16, in which a color burst signal is added to theinput signal. The output signal of the burst signal adder 16 isconverted into an analog signal by the D/A converter 17, and the analogsignal is outputted through the C output terminal 18 similarly to the Ysignal.

FIG. 2 is a detailed block diagram of each of the gamma circuits 5 and13 used in the embodiment shown in FIG. 1. The arrangement shown in FIG.2 includes an input terminal 201, a comparator 202 for comparing aninput signal with a plurality of predetermined values and detectingwhich range corresponds to the range of the input signal, subtractors203 and 207, and decoders 204, 206, 208, 211 and 213 for generatingpredetermined constants "a", "c" and "e" as well as predeterminedcoefficients "b" and "d" in accordance with a selecting signal,respectively. The arrangement also includes coefficient multipliers 205and 210, a multiplier 209 of n₁ bits×n₂ bits, an adder 212, and anoutput terminal 214.

In operation, a signal which has been inputted through the inputterminal 201 is supplied to the comparator 202, in which it is detectedwhich part of a predetermined range corresponds to the range of theinput signal, and a detection signal S_(D) is outputted from thecomparator 202.

The input signal is also supplied to the subtractor 203, in which theinput signal is subtracted from the constant "a" which is generated byand outputted from the decoder 204 in accordance with the detectionsignal S_(D). The obtained difference signal is applied to one inputterminal of the multiplier 209. The difference signal is also suppliedto the coefficient multiplier 205, in which the difference signal ismultiplied by the coefficient "b" which is generated by the decoder 206in accordance with the detection signal S_(D). The signal outputted fromthe coefficient multiplier 205 is subtracted by the subtractor 207 fromthe constant "c" which is similarly generated by the decoder 208, andthe obtained difference signal is applied to the other input terminal ofthe multiplier 209. The output signal of the multiplier 209 is inputtedto the coefficient multiplier 210, in which the input signal ismultiplied by the coefficient "d" which is generated by the decoder 211.The output signal of the coefficient multiplier 210 is inputted to theadder 212, in which the input signal is added to the constant "e" whichis generated by the decoder 213. The resultant signal is outputtedthrough the output terminal 214 to the aforementioned circuit providedat the next stage.

FIGS. 3(a) and 3(b) are explanatory views of the gamma circuit 5 (or 13)shown in FIG. 2. In FIG. 3(a), the horizontal axis represents the inputof the gamma circuit, while the vertical axis represents the output ofthe gamma circuit. If the number of bits of an input signal which isinputted to the gamma circuit is n₃, the values of the input range from0 to 2^(n3) -1. It is assumed here that the range of this input isdivided into k ranges (k=5 in FIG. 3(a)) which are indicated by R₁, R₂,. . . , R_(k), respectively. The maximum value of each of the ranges R₁,R₂, . . . , R₅ is set to a value smaller than 2^(n1) -1 (n₁ representsthe number of bits of either input of the multiplier 209 shown in FIG.2).

For example, if n₃ is 10, the input values range from 0 to 1023. Bydividing this range into five ranges, as shown in FIG. 3(a), and settingthe maximum value of each of these five ranges to a value smaller than255, n₁ =8 can be obtained. In FIG. 3(a), a₁ to a₅ indicate the inputvalues at the starting points of the respective ranges, while e₁ to e₅indicate the corresponding output values, respectively. A single rangeR_(i) selected from the five ranges is shown in FIG. 3(b).

Since the curve y_(i) shown in FIG. 3(b) has an upward convex form andpasses through the origin (a_(i), e_(i)), the curve y_(i) can beapproximated by the following quadratic function:

    (y.sub.i -e.sub.i)=A.sub.i (x-a.sub.i).sup.2 +B.sub.i (x-a.sub.i)(A.sub.i and B.sub.i :predetermined constants)

    (y.sub.i -e.sub.i)=(x-a.sub.i)×{A.sub.i (x-a.sub.i)+B.sub.i }

    y.sub.i =(x-a.sub.i)×{A.sub.i ×(x-a.sub.i)+B.sub.i }+e.sub.i(1)

By changing the values of a_(i), e_(i), A_(i) and B_(i) in accordancewith each range of an input, it is possible to realize thecharacteristic shown in FIG. 3(a). If an actual gamma curve isapproximated by Equation 1, the values of A_(i) and B_(i) becomeextremely small (approximately 2⁻⁵ to 2⁻¹⁵), so that the calculationperformed by the multiplier 209 results in a decrease in the number ofsignificant bits. For this reason, A_(i) and B_(i) are each multipliedby 1/d_(i) (d_(i) <1), and since A_(i) takes negative values for a lineof upward convex form, b_(i) =-A_(i) /d_(i) and c_(i) =B_(i) /d_(i) aresubstituted into Equation 1, the following equation is obtained:

    y.sub.i =(x-a.sub.i)×{-b.sub.i ×(x-a.sub.i)+c.sub.i }×d.sub.i +e.sub.i                                  (2)

A circuit based on Equation 2 is shown in FIG. 2.

The manner of finding A_(i) and B_(i) at this time is as follows. If theranges R₁, R₂, . . . , R₅ shown in FIG. 3(a) are not continuously orsmoothly connected from range to range at the respective dividing points(a_(i), e_(i)), a pseudo-contour will occur in the picture. Therefore,

    y.sub.i (a.sub.i+1)=y.sub.i+1 (a.sub.i+1)                  (continuity condition)

from this continuity condition, the following equation is obtained:

    A.sub.i R.sub.i.sup.2 +B.sub.i R.sub.i=e.sub.i+1           (3)

and

    y.sub.i '(a.sub.i+1)=y.sub.i+1 '(a.sub.i+1)                (smoothness condition)

from this smoothness condition, the following equation is obtained:

    2A.sub.i R.sub.i +B.sub.i =B.sub.i+1                       (4)

B₀ takes a value of the order of 3-4 since B₀ indicates a gain in theneighborhood of x=0. In this case, A₀ is found so as to match a gammacharacteristic. Subsequently, A_(i) and B_(i) are found on the basis ofEquations 3 and 4 as well as the condition of the gamma characteristic,and d_(i) is found so as to match the range of the multiplier 209. Then,b_(i) and c_(i) are found.

It is preferable to construct the coefficient multipliers 205 and 210 byusing a shift computation and an addition for the sake of simplificationof their respective circuit arrangements.

Since d_(i) is used for adjusting the range of the multiplier 209, it ispossible to realize d_(i) by using only a shift computation under thefollowing condition:

    d.sub.i =2.sup.-Pi                                         (P.sub.i :natural number)

Also, the gamma circuit can be simplified by selecting a_(i) within arange of not greater than 2^(n1) -1 so that b_(i) can be realized byusing only a shift computation if possible.

FIG. 4 shows another example of the arrangement of the gamma circuit 5(or 13) shown in FIG. 1. In this example, Equation 2 is modified intothe following equations:

    f.sub.i =a.sub.i ×b.sub.i +c.sub.i                   (5)

    y.sub.i =(x-a.sub.i)×(-b.sub.i x+f.sub.i)×d.sub.i +e.sub.i(6)

and the input of the coefficient multiplier 205 is connected to theinput terminal 201. The constant which is inputted to the subtractor 207is f which is represented by Equation 5.

A second embodiment of the image pickup apparatus according to thepresent invention will be described below with reference to FIGS. 5 to8.

The arrangement shown in FIG. 5 includes the CCD 1 which serves as animage sensor, the sample-and-hold circuit (S/H) 2, the A/D converter(ADC) 3, a gamma circuit 34, an adder 35, a D/A converter (DAC) 36, anoutput terminal 37, a peak detector 38, and a microcomputer 39.

In operation, a subject image (not shown) is formed on the photoelectricconversion surface of the CCD 1 by a photographic optical system (notshown), and is photoelectrically converted into a picked-up imagesignal. The pickup-image signal outputted from the CCD 1 is convertedinto a continuous signal by the sample-and-hold circuit 2, and thecontinuous signal is converted into a digital signal by the A/Dconverter 3. The digital signal outputted from the A/D converter 3 isgamma-corrected by the gamma circuit 34.

The gamma-corrected signal is applied to the adder 35, in which a syncsignal SYNC is added to the gamma-corrected signal. The output signal ofthe adder 35 is converted from digital to analog by the D/A converter36, and the analog signal is outputted via the output terminal 37 toexternal equipment (not shown) as a composite television signal.

The output signal of the A/D converter 3 is also supplied to the peakdetector 38, in which its peak level during one vertical period isdetected. The peak detector 38 is connected to the microcomputer 39 sothat the value of the detected peak level can be read out by themicrocomputer 39. The microcomputer 39 is connected to the gamma circuit34 so that a set value can be written into the gamma circuit 34.

FIG. 6 is a detailed block diagram of the gamma circuit 34 shown in FIG.5. The arrangement shown in FIG. 6 includes a signal input terminal 41,subtractors 42 and 47, decoders 43 and 52 for generating predeterminedcoefficients, multipliers 44 and 50, switches 45, 48 and 54, andregisters 46, 49 and 55 into which to write a set value from themicrocomputer 39. The arrangement also includes a coefficient multiplier51, an adder 53, an output terminal 56, a comparator 57 for comparing aninput signal with a plurality of predetermined values and outputting adetection signal S_(D) which indicates the range of the input signal,and a setting signal input terminal 58.

In operation, an input signal which has been inputted through the signalinput terminal 41 is supplied to the subtractor 42. In the subtractor42, a constant "a" which is generated by the decoder 43 in accordancewith the detection signal S_(D) is subtracted from the input signal.Then, the obtained difference signal is supplied to the multiplier 44,in which it is multiplied by a value "b" outputted from the switch 45which is arranged to select any one of predetermined values 0, b₁ and avalue b₂ of the register 46 in accordance with the detection signalS_(D) and output the selected value as the value "b". The output signalof the multiplier 44 is supplied to the subtractor 47, in which thesignal is subtracted from a value "c" outputted from the switch 48 whichis arranged to select any one of predetermined values c₁, c₂ and a valuec₃ of the register 49 in accordance with the detection signal S_(D) andoutput the selected value as the value "c".

The output signal of the subtractor 47 is multiplied by the outputsignal of the subtractor 42 in the multiplier 50, and, in thecoefficient multiplier 51, the output signal of the multiplier 50 ismultiplied by a value "d" which is generated by the decoder 52 inaccordance with the detection signal S_(D). Then, in the adder 53, theoutput signal of the coefficient multiplier 51 is added to a value "e"which is outputted from the switch 54 which is arranged to select anyone of predetermined values e₁, e₂ and a value e₃ of the register 55 inaccordance with the detection signal S_(D) and output the selected valueas the value "e". The output signal of the adder 53 is outputted to theadder 35 of FIG. 5 through the output terminal 56.

The input signal which has been inputted through the signal inputterminal 41 is also supplied to the comparator 57, in which the inputsignal is compared with the plurality of predetermined values. Thecomparator 57 generates the detection signal S_(d) and applies it toeach part in the above-described manner.

Also, values specified by the microcomputer 39 are respectively writteninto the registers 46, 49 and 55 by a setting signal which has beeninputted through the setting signal input terminal 58.

FIGS. 7(a) and 7(b) are explanatory views of the operation of the secondembodiment. FIG. 7(a) shows the input-output characteristic of the gammacircuit 34. The values of the input signal range from 0 to x_(m), whilethe values of the output signal range from 0 to y_(m). These ranges areeach divided into three ranges R₁ (0 to a₂), R₂ (a₂ to a₃) and R₃ (a₃ tox_(m)). The gamma characteristic is approximated by a predeterminedquadratic function y₁ in the range R₁ ; by a variable quadratic functiony₂ in the range R₂ ; and by a variable linear function y₃ in the rangeR₃. y₁, y₂ and y₃ are expressed as follows:

    y.sub.1 =x×(b.sub.1 x+c.sub.1)                       (7)

    y.sub.2 =(x-a.sub.2)×(b.sub.2 (x-a.sub.2)+c.sub.2)+e.sub.2(8)

    y.sub.3 =(x-a.sub.3)×c.sub.3 +e.sub.3                (9)

To connect the functions Y₁ and Y₂ continuously and smoothly from rangeto range in FIG. 7(a), the following equations are obtained:

    from y.sub.1 (a.sub.2)=y.sub.2 (a.sub.2)

    a.sub.2.sup.2 b.sub.1 +a.sub.2 c.sub.1 =e.sub.2            (10)

    from y.sub.1 '(a.sub.2)=y.sub.2 '(a.sub.2)

    2a.sub.2 b.sub.1 +c.sub.1 =c.sub.2                         (11)

Similarly, in the case of the functions y₂ and y₃,

    from y.sub.2 (a.sub.3)=y.sub.3 (a.sub.3)

    (a.sub.3 -a.sub.2).sup.2 b.sub.2 +(a.sub.3 -a.sub.2)C.sub.2 =e.sub.3(12)

    from y.sub.2 '(a.sub.3)=y.sub.3 '(a.sub.3)

    2(a.sub.3 -a.sub.2)b.sub.2 +c.sub.2 =e.sub.3               (13)

To satisfy Equations 10 and 11, c₂ and e₂ are fixed values. At thistime, by ma king b₂ variable, the value of y₂ (a₃) can be varied. It canalso be seen from Equations 12 and 13 that if y₂ is determined, y₃ isprimarily determined.

Accordingly, to vary the gamma characteristic as shown by l₁ and l₂ inFIG. 7(a), it is only necessary to find the corresponding c₃ and e₃ fromEquations 12 and 13 while using b₂ as a parameter.

FIG. 7(b) is a graphic representation showing how b₂ is made to vary inaccordance with a detected peak level as well as how c₃ and e₃ vary withthe variation of b₂. In FIG. 7(b), since b₂ takes a negative value, b₂indicates -b₂. If the peak level is lower than a predetermined value P₁,-b₂ is set to a predetermined value. If the peak level exceeds P₁, -b₂gradually increases with the increase of P₁, and if the peak level isnot lower than P₂, -b₂ is set to another predetermined value. If -b₂increases excessively, the inclination of y₂ becomes negative midway.For this reason, -b₂ is not increased if the peak level is not lowerthan P₂.

As can be seen from Equations 12 and 13, c₃ and e₃ vary with thevariation of the value of b₂ as shown in FIG. 7(b). In an actualcircuit, the number of significant digits decreases due to acancellation of significant digits in the multiplier 50. Therefore,b_(i) and c_(i) are each multiplied by 1/d (d<1), and the polarity ofb_(i) is reversed because of its negative polarity to obtain thefollowing equations:

    y.sub.1 =x×(-b.sub.1 x+c.sub.1)×d.sub.1

    y.sub.2 =(x-a.sub.2)×(-b.sub.2 x+c.sub.2)×d.sub.2 +e.sub.2

    y.sub.3 =c.sub.3 (x-a.sub.3)×d.sub.3 +e.sub.3

FIG. 8 is a flowchart showing the operation of the microcomputer 39 usedin the second embodiment.

The flow starts in Step 101. In Step 102, the flow waits until avertical sync pulse VD is inputted. If the vertical sync pulse VD isinputted, the microcomputer 39 reads a peak level from the peak detector38 in Step 103. Then, in Step 104, on the basis of the value of the readpeak level, b₂ is found from the table shown in FIG. 7(b). In Step 105,C₃ and e₃ are found by using Equations 12 and 13, and, in Step 106, theobtained b₂, c₃ and e₃ are respectively written into the registers 46,49 and 55. The flow returns to Step 102.

Incidentally, in the second embodiment, the peak detector 38 may alsouse a maximum value detecting circuit and a minimum value detectingcircuit to find the difference between maximum and minimum values andobtain contrast information so that the gamma characteristic can bevaried. Otherwise, it is also possible to employ information on thedegree of opening of a diaphragm, the difference between the maximum andminimum values of the values obtained by dividing one picture into nareas and individually integrating the n areas, or white balanceinformation.

FIG. 9 is a schematic block diagram showing a third embodiment of theimage pickup apparatus according to the present invention.

The arrangement shown in FIG. 9 includes a CCD 301 which serves as animage sensor, a sample-and-hold circuit 302 for converting the outputsignal of the CCD 301 into a continuous signal, an A/D converter 303, alow-pass filter 304, a gamma circuit 305 having characteristic varyingterminals and capable of varying its characteristic, an adder 306, a D/Aconverter 307, and an output terminal 308.

The low-pass filter 304, the gamma circuit 305 and the adder 306 areeach formed by a digital circuit.

In operation, a subject image (not shown) is formed on the photoelectricconversion surface of the CCD 301 by a photographic optical system (notshown), and is photoelectrically converted into a video signal. Thevideo signal is converted into a continuous signal by thesample-and-hold circuit 302, and the continuous signal is converted intoa digital video signal by the A/D converter 303. The digital signal islimited to a necessary band by the low-pass filter 304. The outputsignal of the low-pass filter 304 is supplied to the gamma circuit 305,in which the signal is subjected to gamma correction of characteristicaccording to coefficients K1 and K2 inputted to the two characteristicvarying terminals of the gamma circuit 305. The output signal of thegamma circuit 305 is supplied to the adder 306, in which a sync signalis added to the signal. The output signal of the adder 306 is convertedinto an analog signal by the D/A converter 307, and the analog signal isoutputted via the output terminal 308 to external equipment (not shown)such as a VTR or a television set.

FIG. 10 is an explanatory view of the operation of the gamma circuit 305shown in FIG. 9. In FIG. 10, the horizontal axis x and the vertical axisy represent the input and the output of the gamma circuit 305,respectively.

The value of an output y₁ indicates a 100% white level, and the signallevel outputted from the output terminal 308 at this time becomes thereference white level of a television signal.

The value of an output y₂ indicates a white clip level, and the signallevel outputted from the output terminal 308 at this time becomes thewhite clip level of a television signal.

x₁ and x₂ are inputs corresponding to y₁ and y₂, respectively. x₁ and x₂normally indicate the standard output level and the saturation outputlevel of the CCD 301, respectively, and x₂ =2x₁ to 5x₁. In FIG. 10, x₂=2x₁ by way of explanation. In general, the range between 0 and x₁ iscalled a gamma region, while the range between x₁ and x₂ is called aknee region.

If the coefficient K1 shown in FIG. 9 is made variable, the gammacharacteristic varies within the range of the inputs 0 to x₁ and withinthe range of the outputs 0 to y₁, as shown in FIG. 10. In this case, thegamma characteristic curve varies in such a manner as to necessarilypass through the point (0, 0) and the point (x₁, y₁). If the coefficientK1 is set to a predetermined value, a standard gamma characteristic(y=x⁰.45) is obtained. As K1 is made larger, the curvature of the curvebecomes larger as shown in FIG. 10, while as K1 is made smaller, thecurve progressively approaches a straight line.

Also, if the coefficient K2 shown in FIG. 9 is made variable, the gammacharacteristic varies within the range of the inputs x₁ to x₂ and withinthe range of the outputs y₁ and y₂, as shown in FIG. 10. In this case,the gamma characteristic curve varies in such a manner as to necessarilypass through the point (x₁, y₁). If the coefficient K2 is set to apredetermined value, the curve becomes a straight line which passesthrough the point (x₂, y₂). As K2 is made larger, the inclination of thecurve becomes larger, while as K2 is made smaller, the inclinationbecomes smaller. If the value of y is larger than y₂, the curve isclipped at y₂.

FIG. 11 is a detailed block diagram of the gamma circuit 305 shown inFIG. 9. The gamma circuit 305 includes a switch 310 responsive to acontrol signal S1 for performing switching between a path along which topass an input signal and a path along which to output a predeterminedvalue x₁, a coefficient multiplier 311 for multiplying the input signalby a predetermined coefficient u=y₁ /x₁, an adder 312, a functioncircuit 313 for generating a function according to a control signal S2,a multiplier 314, a comparator 315 for comparing the input signal with apredetermined value and outputting the predetermined control signals S1and S2 according to the comparison result, a switch 316 for performingswitching in accordance with the control signal S1, and registers 317and 318 for holding the values of the respective coefficients K1 and K2.

In operation, an input signal is first compared with the predeterminedvalue by the comparator 315, and the comparator 315 outputs the controlsignal S1 to the switches 310 and 316 and the control signal S2 to thefunction circuit 313 in accordance with the comparison result. If S1=0,the input signal passes through the switch 310 and is multiplied by thepredetermined value u by the coefficient multiplier 311, and the outputsignal of the coefficient multiplier 311 is applied to one input of theadder 312. Also, according to the input signal, a predetermined functionis generated by the function circuit 313, and in the multiplier 314 thevalue of the predetermined function is multiplied by the coefficient K1selected by the switch 316. The output signal of the multiplier 314 isadded to the output signal of the coefficient multiplier 311 in theadder 312, and the addition result is outputted from the adder 312.

If S1=1, the predetermined value x₁ is outputted from the switch 310 andis multiplied by the predetermined value u by the coefficient multiplier311. The multiplication result is supplied to the adder 312 as y₁. Also,in the multiplier 314, the output signal of the function circuit 313 ismultiplied by the coefficient K2 selected by the switch 316. Themultiplication result is added to y₁ in the adder 312, and the additionresult is outputted from the adder 312.

FIGS. 12(a), 12(b) and 12(c) are explanatory views of the operation ofthe gamma circuit 305 of FIG. 11.

In FIG. 12(a), the required gamma characteristic is shown as beingdivided into straight lines l₁ and l₂ and curves (hatched parts) C1 andC2. In this case,

    l.sub.1 :y=(y.sub.1 /x.sub.1)x=ux (0≦x ≦x.sub.1)

    l.sub.2 :y=y.sub.1 (x.sub.1 ≦x≦x.sub.2)

The straight lines l₁ and l₂ are connected to each other at the point(x₁, y₁). The straight lines l₁ and l₂ are each created by thecomparator 315, the switch 310 and the coefficient multiplier 311. Also,the values of the respective curves C1 and C2 are "0" at the points ofx=0 and x=x₁. FIG. 12(b) shows only the curves C1 and C2 taken from FIG.12(a). The characteristic of the function circuit 313 is selected so asto generate the characteristic shown in FIG. 12(b). FIG. 12(c) shows thecombined input-output characteristic of the function circuit 313 and themultiplier 314. The characteristic varies within the range of 0 to x₁ inaccordance with the coefficient K1 and within the range of x₁ to x₂ inaccordance with the coefficient K2. By adding together the outputcharacteristic shown in FIG. 12(c) and the characteristics of the switch310 and the coefficient multiplier 311 shown in FIG. 12(a), theinput-output characteristic of FIG. 10 is obtained.

FIG. 13 is a detailed block diagram of the function circuit 313 shown inFIG. 11. The function circuit 313 includes a subtractor 401, switches402, 407, 410 and 414 which are switchable by the control signal S2,decoders 403, 404, 405, 408, 409, 411, 415, 416 and 417 for outputtingvalues a₁, a₂, a₃, b₁, b₂, b₃, c₁, c₂ and c₃, respectively, adders 406and 413, and a multiplier 412.

In operation, when an input signal is supplied to the subtractor 401,the switch 402 selects any one value from the values a₁, a₂ and a₃ inaccordance with the control signal S2, and the selected value issubtracted from the input signal in the subtractor 401. The outputsignal of the subtractor 401 is supplied to the adder 406, in whicheither one of the values b₁, and b₂ which has been selected by theswitch 407 is added to the signal. The switch 410 selects either one ofthe output signal of the adder 406 and the value b₃, and the selectedone is multiplied by the output signal of the subtractor 401 in themultiplier 412. In the adder 413, the output signal of the multiplier412 is added to any one of the values c₁, c₂ and C₃ which has beenselected by the switch 414. The addition result is outputted from theadder 413.

FIG. 14 is an explanatory view of the operation of the function circuit313 shown in FIG. 13. As shown in FIG. 14, the characteristic shown inFIG. 12(b) is divided into three ranges of 0 to x₃, X₃ to x₁ and x₁ tox₂, and the aforementioned comparator 315 generates the control signalS2 which satisfies the following relationships:

0≦x<x₃ →S2=1

x₃ ≦x<x<x₁ →S2=2

x₁ ≦x→S2=3

The range of 0 to x₃ is represented by a first quadratic curve C11:

    y=(x-a.sub.1)×((x-a.sub.1)+b.sub.1)+c.sub.1

The range of X₃ to x₁ is represented by a second quadratic curve C12:

    y=(x-a.sub.2)×((x-a.sub.2)+b.sub.2)+c.sub.2

The range of x₁ to x₂ is represented by a straight line C2:

    y=(x-a.sub.3)×b.sub.3 +c.sub.3

It is preferable to select each of the above coefficients so that therequired gamma characteristic can be approximated under the followingconditions:

C11: Passes through (0, 0).

C12: Passes through (x₁, 0).

Takes the same value as C11 at x₃.

Takes the same differential coefficient as C11 at x₃ (smoothly joins toC11).

C2 : Passes through (x₁, 0).

By using the thus-obtained individual coefficients in the circuit ofFIG. 13, it is possible to provide the function circuit 313 having thecharacteristic of FIG. 12(b).

FIG. 15 is a functional block diagram schematically showing a fourthembodiment of the image pickup apparatus according to the presentinvention.

The arrangement shown in FIG. 15 includes a CCD 501 which serves as animage sensor, a sample-and-hold circuit 502 for converting the outputsignal of the CCD 501 into a continuous signal, an A/D converter 503, alow-pass filter 504 for forming a luminance signal, a gamma correctioncircuit 505, a clipping circuit 506 for clipping black and white levels,a sync signal adder 507, a D/A converter 508, a video signal outputterminal 509, a switch 510 for generating a selecting signal SW forselecting a gamma characteristic, and a gamma controlling circuit 511for generating a control signal MW for controlling a gammacharacteristic, the gamma controlling circuit 511 being made up of amicrocomputer and associated elements.

In operation, a subject image (not shown) is formed on the photoelectricconversion surface of the CCD 501 by a photographic optical system (notshown), and is photoelectrically converted into a picked-up imagesignal. The output signal of the CCD 501 is converted into a continuoussignal by the sample-and-hold circuit 502, and the continuous signal isconverted into a digital picked-up image signal by the A/D converter503.

This digital picked-up image signal is converted into a luminance signalby the low-pass filter 504, and the output signal of the low-pass filter504 is subjected to gamma correction according to the control signal MWfor controlling a gamma characteristic which will be described later, inthe gamma correction circuit 505. The output signal of the gammacorrection circuit 505 is supplied to the clipping circuit 506, in whichthe levels of the output signal which are lower than a predeterminedvalue or the levels of the output signal which are higher than apredetermined value are clipped. A sync signal is added to the outputsignal of the clipping circuit 506 by the sync signal adder 507, therebyforming a digital video signal. The digital video signal is convertedinto an analog video signal by the D/A converter 508. The analog videosignal is outputted vie the video signal output terminal 509 to externalequipment (not shown) such as a television set or a VTR. The switch 510generates the selecting signal SW for a gamma characteristic inaccordance with a position selected by an operator, and the gammacontrolling circuit 511 generates the control signal MW for controllingthe gamma characteristic in accordance with the selecting signal SW,whereby the characteristic of the gamma correction circuit 505 is variedas described above.

FIG. 16 is a detailed diagram of the gamma correction circuit 505 shownin FIG. 15.

The gamma correction circuit 505 includes a signal input terminal 601, acomparator 602 for comparing an signal input I with reference valuesinputted to individual reference inputs R1, R2, R3 and R4 and outputtingthe comparison result, memories 603, 604, 605, 606 and 607 which holdpredetermined values M11, M12, M13, M14 and M15, an input terminal 608for input of the gamma characteristic control signal MW, subtractors 609and 623, switch circuits 610, 616, 622, 624, 636 and 637, coefficientmultipliers 611, 612, 613, 614, 615, 617, 618, 619, 620, 621, 631, 632,633, 634 and 635 which respectively have coefficients K11, K12, K13,K14, K15, K21, K22, K23, K24, K25, K31, K32, K33, K34 and K35, memories625, 626, 627, 628, 629, 638, 639, 640, 641 and 642 which respectivelyhold values M21, M22, M23, M24, M25, M31, M32, M33, M34 and M35 each ofwhich is writable by the control signal MW, a multiplier 630, an adder643, and a signal output terminal 644.

As described previously, the luminance signal outputted from thelow-pass filter 504 is inputted via the signal input terminal 601 as aninput signal x. The input signal x is first inputted to the comparator602. The comparator 602 compares the signal x inputted via the inputterminal 601 with each of the values M12, M13, M14 and M15 inputted tothe respective reference inputs R1, R2, R3 and R4, and generates acomparison output signal SC. The switch circuits 610, 616, 622, 624, 636and 637 switch in accordance with the comparison output signal SC. Forexample, if the input signal I is smaller than the input value M12 atthe reference input R1, the comparator 602 generates SC=1, whereby eachof the switch circuits 610, 616, 622, 624, 636 and 637 switches to itsposition "1". If the input signal I is greater than the input value M12at the reference input R1 and is smaller than the input value M13 at thereference input R2, the comparator 602 generates SC=2, whereby each ofthe switch circuits 610, 616, 622, 624, 636 and 637 switches to itsposition "2".

The input signal x is also inputted to the subtractor 609, in which anyone value of the predetermined values M11 through M15 which has beenselected by the switch circuit 610 in accordance with the signal SC issubtracted from the input signal x. The output signal of the subtractor609 is first multiplied by the predetermined coefficients K11 throughK15 in the respective coefficient multipliers 611 through 615. Any onesignal is selected from the output signals of the respective coefficientmultipliers 611 through 615 by the switch circuit 616 in accordance withthe signal SC. The output signal of the switch circuit 616 is inputtedto the multiplier 630. The output signal of the subtractor 609 is alsomultiplied by the predetermined coefficients K21 through K25 by therespective coefficient multipliers 617 through 621. Any one signal isselected from the output signals of the respective coefficientmultipliers 617 through 621 by the switch circuit 622 in accordance withthe signal SC. In the subtractor 623, the selected signal is subtractedfrom any one value which has been selected from the values M21 throughM25 held in the respective memories 625 through 629 by the switchcircuit 624 in accordance with the signal SC. The signal output of thesubtractor 623 is applied to the other input of the multiplier 630.

The multiplier 630 multiplies the signal inputted from the switchcircuit 616 by the output signal of the subtractor 623. The outputsignal of the multiplier 630 is multiplied by the predeterminedcoefficients K31 through K35 by the respective coefficient multipliers631 through 635. Any one signal is selected from the output signals ofthe respective coefficient multipliers 631 through 635 by the switchcircuit 636 in accordance with the signal SC. The output signal of theswitch circuit 636 is, in the adder 643, added to any one value whichhas been selected from the values M31 through M35 held in the respectivememories 638 through 642 by the switch circuit 637 in accordance withthe signal SC. The output signal of the adder 643 is inputted via theoutput terminal 644 to the clipping circuit 506 as described below.

In the meantime, when the gamma characteristic control signal MW isinputted via the input terminal 608 for input of the gammacharacteristic control signal MW, the values held in the respectivememories 625, 626, 627, 628, 629, 638, 639, 640, 641 and 642 arerewritten according to the value of the gamma characteristic controlsignal MW.

FIGS. 17(a), 17(b)and 17(c) are explanatory views of the operation ofthe gamma correction circuit 505 shown in FIG. 15, which is used in thefourth embodiment of the present invention.

FIG. 17(a) shows the input-output characteristic of the gamma correctioncircuit 505. As shown in FIG. 17(a), the input-output characteristicconsists of a combination of five curves A to E on the basis of thevalues M12 to M15 held in the respective memories 604 to 607 shown inFIG. 16. For each of the intervals, the signal SC takes any one valuefrom the values of 1 to 5 in the above-described manner. The switchcircuits 610, 616, 622, 624, 636 and 637 perform switching according tothe value of the signal SC, whereby the characteristics of therespective curves vary. If the characteristic of an n-th curve isdenoted by yn(x), computation performed by the gamma correction circuit505 show in FIG. 16 is represented as the quadratic equation:

    yn(x)= (x-M1n)*K1n* -(x-M1n)*K2n)!*K3n+M3n                 (1)

In Equation 1, n represents the value of the signal SC. Bydifferentiating this equation, the following equations are obtained:

    yn'(x)=K1n*M2n*K3n-2*(x-M1n)*K1n*K2n*K3n                   (2)

For x=M1n,

    yn(M1n)=M3n                                                (3)

    yn'(M1n)=K1n*M2n*K3n                                       (4)

For x=M1(n+1),

    yn(M1(n+1))= (M1(n+1)-M1n)*K1n*(-(M1(n+1)-M1n) *K2n+M2n)!*K3n+M3n(5)

    yn'(x)=K1n*M2n*K3n-2*(M1(n+1)-M1n) *K1n*K2n*K3n            (6)

In the gamma correction characteristic shown in FIG. 17(a), to preventoccurrence of a degradation in image quality, the n-th curve and the(n+1)-th curve must be smoothly joined together. The conditions requiredto realize such smooth connection at the connection part x=M1(n+1) areas follows:

    yn(M1(n+1))=yn+1(M1(n+1))                                  (7)

    y'n(M1(n+1))=y'n+1(M1(n+1))                                (8)

These conditions are shown in FIG. 17(b).

To satisfy the conditions, the following equations are obtained fromEquations 3, 4, 5 and 6:

     (M1(n+1)-M1n)*K1n*(-(M1(n+1) -M1n)+K2n+M2n)!*K3n+M3n=M3(n+1)(9)

    K1n*M2n*K3n-2*(M1(n+1)-M1n)*K1n*K2n*K3n =k1(n+1)*M2(n+1)*K3(n+1)(10)

From Equations 9 and 10, it can be seen that a curve yn+1(x) whichsmoothly joins to an arbitrary curve yn(x) can be obtained by varyingonly M2(n+1) and M3(n+1) in the computational processing of FIG. 16.

FIG. 17(c) shows input-output characteristics obtainable when the gammacorrection characteristic is made variable. In general, if the gammacorrection characteristic is made variable, the characteristic of aninput signal in the neighborhood of 0 becomes most important. If theeffect of gamma correction is to be reduced, the gain in theneighborhood of 0 is made closer to 1, whereas if the effect of gammacorrection is to be increased, the gain in the neighborhood of 0 isincreased. In FIG. 17(c), (1) denotes a curve which is obtained when theeffect of gamma correction is made greater than a standard level, (2)denotes a standard curve, and (3) denotes a curve which is obtained whenthe effect of gamma correction is made smaller than the standard level.G1, G2 and G3 denote the inclinations of the respective curves (1), (2)and (3) in the neighborhood of 0 and satisfy the condition of G1>G2>G3.

The aforementioned M21 to M25 and M31 to M35 for each of the curves canbe determined in the following manner.

To determine, for example, the curve (1), M21 and M31 are first foundfrom Equations 3 and 4:

    M31=0                                                      (11)

    M21=G1/K11/K31                                             (12)

These M21 and M31 are respectively employed to find sequentially M22 toM25 and M32 to M35 from Equations 9 and 10, whereby the value of each ofthe memories for the characteristic of the curve (1) can be obtained. Inthis case, even if the left and right sides of each of Equations 9 and10 do not completely coincide, there is no substantial problem as far asthe amount of discrepancy therebetween is within a visually allowablerange. For example, in the case of Equation 9, if an error is notgreater than approximately 0.4% of a gamma-corrected output signal,there is no substantial problem in visual terms. In the case of Equation10, because of its slightly wider allowable range, there is no problemas far as an error is not greater than approximately 10% of agamma-corrected output signal. Accordingly, it suffices to find therespective values of the memories while taking such errors into account,so as to cause a gamma correction characteristic to coincide with anappropriate characteristic as accurately as possible.

The thus-obtained values are written into a ROM incorporated in thegamma controlling circuit 511. The above-described gamma correctioncharacteristic can be obtained by writing the values into each of thememories in accordance with the selecting signal SW outputted from theswitch 510.

FIG. 18 is a flowchart showing the operation of gamma controllingcircuit 511 which is formed by, for example, a microcomputer.

The flow starts in Step 701, and a selecting signal SW is read in Step702. In Step 703, the values M21 to M35 according to the read SW arefound through the computations of Equations 11, 12, 9 and 10 by using again G close to 0 according to SW, or calculated values, which arebeforehand written in a ROM incorporated in the microcomputer, are readout according to SW and the read values are outputted as a gammacharacteristic control signal MW. The thus-obtained values M21 to M35are written into the memories 625 to 629 and 638 to 642 in thepreviously-described manner. Then, in Step 704, SW0=SW is set and, inStep 705, SW is again read. In Step 708, SW is compared with SW0. IfSW=SW0, the process returns to Step 705, and Steps 705 and 706 are againexecuted. If it is determined in Step 706 that SW is not equal to SW0,the process returns to Step 703, in which the values M21 to M35 arefound according to SW, which was again read in Step 705, and outputtedas MW. Then, Step 704 and the following steps are executed.

FIG. 19 is a detailed diagram of a second example of the gammacorrection circuit 505. In FIG. 19, the same reference numerals are usedto denote portions identical to or equivalent to those shown in FIG. 16.An input signal is processed in a manner similar to that described abovewith reference to FIG. 16, whereby a gamma-corrected output signal y isformed. Also, values according to a gamma correction control signal MWwhich is inputted through the input terminal 608 are respectivelywritten into and held in the memories 603 to 607, 625 to 629 and 638 to642. Accordingly it is possible to vary the values M11 to M15 inaddition to the values which are variable in the example shown in FIG.16. With this arrangement, it is possible to widen the range ofvariation of a gamma correction characteristic to a further extent. Inthe arrangement shown in FIG. 16, if the M gamma correctioncharacteristic is greatly varied with M11 to M15 fixed, the requiredcharacteristic may not be obtained or the multiplier 630 may overflow.For this reason, the example shown in FIG. 19 is arranged in such amanner that the values M11 to M15 can vary with SW.

In the above-described fourth embodiment, since the coefficientmultipliers 611 to 615, 617 to 621 and 631 to 635 are inserted toprevent the multiplier 630 from overflowing, the values of therespective coefficient multipliers may be arranged in the form of two tothe n-th power, such as "1", "2", "3", "4" or "8", or in the form of acombination of approximately two or three numbers selected from thesenumbers. Accordingly, an extremely simple arrangement can be adopted.Further, in this arrangement, since it is possible to effectivelyutilize the dynamic range of the multiplier 630 having a large circuitscale, it is possible to reduce the number of input and output bits ofthe multiplier 630 and hence the circuit scale thereof. Accordingly, itis possible to prevent the multiplier 630 from producing an error due toa cancellation of significant digits. If another arrangement including amultiplier having a fully large dynamic range is adopted, part or thewhole of the coefficient multipliers shown in FIG. 16 or 19 can beomitted.

Although FIGS. 17(a), 17(b) and 17(c) show the gamma correctioncharacteristic corresponding to the 0 to 100% range of an input, acharacteristic corresponding to 100% or more of an input, which iscalled knee characteristic, can be similarly realized. In this case, itis possible to adopt an arrangement in which the knee characteristic isvaried according to SW on the basis of the settings of the values M11 toM35 of the respective memories, or it is also possible to adopt anarrangement in which the knee characteristic does not greatly vary evenif SW is varied.

The above-described fourth embodiment is arranged in such a manner thatthe gamma correction characteristic is formed by the five curves.However, this arrangement is not to be construed as a limiting example.For example, it is possible to adopt an arrangement in which the gammacorrection characteristic is formed by switching two or more arbitrarycurves.

The above-described fourth embodiment is also arranged in such a mannerthat values are written into the memories 603 to 607, 625 to 629 and 638to 642 in accordance with the gamma characteristic control signal MW.However, it is also possible to adopt an arrangement which has memoriesin which a plurality of predetermined values are beforehand written. Inthis arrangement, any one selected from the predetermined values isoutputted from each of the memories in accordance with the gammacharacteristic control signal MW.

Although in the above-described fourth embodiment the function expressedas Equation 1 is realized as a gamma curve, this function is not to beconstrued as a limiting example. For example, various functions havingdifferent characteristic curves of upward convex form may be prepared,and a gamma curve may be formed by switching the various functions. Inthis case, by varying constants in the functions so that Equations 7 and8 can be valid at the connection points of the functions, it is possibleto realize a gamma correction circuit of variable characteristicaccording to the present invention.

FIG. 20 is a block diagram schematically showing a fifth embodiment ofthe present invention.

The arrangement shown in FIG. 20 includes a CCD 801 which serves a s animage sensor, a sample-and-hold circuit 802 for converting the outputsignal of the CCD 801 into a continuous signal, an amplifier 803 thegain of which can be varied in accordance with a gain variation controlinput, a memory 804 for memorizing a gain control value G1, an A/Dconverter 805, a low-pass filter 806 for forming a luminance signal, anadder 807, a memory 808 for holding a value L1, a gamma correctioncircuit 809, an adder 810, a memory 811 for holding a value L2, amultiplier 812, a memory 813 for holding a value G2, a limiter 814 forlimiting a black level and a white level, a D/A converter 815, atelevision signal output terminal 816, and a switch 817 for generating amemory control signal S for controlling the values of the respectivememories 804, 808, 811 and 813.

In operation, a subject image (not shown) is formed on the image sensingsurface of the CCD 801 and photoelectrically converted into a picked-upimage signal. The picked-up image signal outputted from the CCD 801 isconverted into a continuous signal by the sample-and-hold circuit 802.The output signal of the sample-and-hold circuit 802 is amplified by theamplifier 803 by using a gain according to the value G1 held in thememory 804. The output signal of the amplifier 803 is converted into adigital picked-up image signal by the AID converter 805.

This digital picked-up image signal is converted into a luminance signalby the low-pass filter 806, and the output luminance signal of thelow-pass filter 806 is applied to the adder 807. In the adder 807, thevalue L1 held in the memory 808 is added to the luminance signal. Theoutput signal of the adder 807 is subjected to gamma correction in thegamma correction circuit 809, and the output signal of the gammacorrection circuit 809 is applied to the adder 810. In the adder 810,the value L2 held in the memory 811 is added to the applied signal, andin the multiplier 812 the output signal of the adder 810 is multipliedby the value G2 held in the memory 813. The output signal of themultiplier 812 is supplied to the limiter 814, in which the levels ofthe output signal which are lower than a predetermined value or thelevels of the output signal which are higher than a predetermined valueare limited. The output signal of the limiter 814 is converted fromdigital to analog by the DIA converter 815, and the analog video signalis outputted via the output terminal 816 to external equipment (notshown) such as a television set or a VTR. The switch 817 generates thememory control signal S in accordance with a position selected by anoperator, thereby controlling th e memories 804, 808, 811 and 813 tovary the respective values held in them. By varying the respectivevalues of the memories 804, 808, 811 and 813, the gamma correctioncharacteristic can be varied through the overall processing performed inthe entire apparatus as will be described later.

Each of the memories 804, 808, 811 and 813 may be formed by a ROM inwhich predetermined values are written, and any one selected from thepredetermined values may be outputted from each of the memories 804,808, 811 and 813 in accordance with the memory control signal S.Otherwise, each of the memories 804, 808, 811 and 813 may be formed by aRAM, and any one selected from the values memorized in another memorymay be written into each of the memories 804, 808, 811 and 813 inaccordance with the memory control signal S.

The gamma correction characteristic can be varied far more finely orcontinuously. To realize this operation, it is only necessary toconstruct the switch 817 of a potentiometer or an up-down switch and anup-down counter. The operator can operate the switch 817 to adjust thegamma correction characteristic to the desired characteristic.

FIGS. 21(a) and 21(b) are explanatory views of the operation of thefifth embodiment.

FIG. 21(a) shows the input-output characteristic of the gamma correctioncircuit 809, The characteristic of the gamma correction circuit 809 isset so that, by using a part B-D, it is possible to obtain a gammacorrection characteristic which is in a normal state. Also, by using apart A-D, it is possible to obtain a gamma correction characteristicwhich provides a larger amount of gamma correction than in the normalstate: That is to say the gain in the neighborhood of a black level ishigher than in the normal state, while the gain in the neighborhood of awhite level is lower than in the normal state. Also, by using a partC-D, it is possible to obtain a gamma correction characteristic whichprovides a smaller amount of gamma correction than in the normal state:That is to say the gain in the neighborhood of a black level is lowerthan in the normal state, while the gain in the neighborhood of a whitelevel is higher than in the normal state. Also, a part D-E constitutes aknee characteristic which is a compression characteristic for a highluminance level.

If the gamma correction characteristic in the normal state is to beobtained, the gain control value G1 is set to a value which provides again K1 which enables ((100% white level)-(black level)) at the outputterminal of the CCD 801 to become (x4-x2) at the input of the gammacorrection circuit 809. Further, the value Li is set to x2, the value L2is set to -y2, and the value G2 is set to a value which enables (y4-y2)to become ((100% white level)-(black level)) at the output terminal 816.In this arrangement, the black level of the output signal of the CCD 801passes through the point (x2, y2) shown in FIG. 21(a) and constitutesthe black level at the output terminal 816, whereas the 100% white levelof the output signal of the CCD 801 passes through the point (x4, y4)shown in FIG. 21(a) and constitutes the 100% white level at the outputterminal 816. Accordingly, as described above, by using thecharacteristic of the part B-D, it is possible to obtain the gammacorrection characteristic which is in the normal state.

If the gamma correction characteristic which provides a larger amount ofgamma correction than in the normal state is to be obtained, the gaincontrol value G1 is set to a value which provides a gain K2 whichenables ((100% white level)-(black level)) at the output terminal of theCCD 801 to become (x4-x1) at the input of the gamma correction circuit809. Further, the value L1is set to x1, the value L2 is set to -y1, andthe value G2 is set to a value which enables (y4-y1) to become ((100%white level) (black level)) at the output terminal 816. In thisarrangement, the black level of the output signal of the CCD 801 passesthrough the point (x1, y1) shown in FIG. 21(a) and constitutes the blacklevel at the output terminal 816, whereas the 100% white level of theoutput signal of the CCD 801 passes through the point (x4, y4)shown inFIG. 21(a) and constitutes the 100% white level at the output terminal816. Accordingly, as described above, by using the characteristic of thepart A-D, it is possible to obtain the gamma correction characteristicwhich provides a larger amount of gamma correction than in the normalstate.

If the gamma correction characteristic which provides a smaller amountof gamma correction than in the normal state is to be obtained, the gaincontrol value G1 is set to a value which provides a gain K3 whichenables ((100% white level)-(black level)) at the output terminal of theCCD 801 to become (x4-x3) at the input of the gamma correction circuit809. Further, the value L1 is set to x3, the value L2 is set to -y3, andthe value G2 is set to a value which enables (y4-y3) to become ((100%white level)-(black level)) at the output terminal 816. In thisarrangement, the black level of the output signal of the CCD 801 passesthrough the point (x3, y3) shown in FIG. 21(a) and constitutes the blacklevel at the output terminal 816, whereas the 100% white level of theoutput signal of the CCD 801 passes through the point (x4, y4) shown inFIG. 21(a) and constitutes the 100% white level at the output terminal816. Accordingly, as described above, by using the characteristic of thepart C-D, it is possible to obtain the gamma correction characteristicwhich provides a smaller amount of gamma correction than in the normalstate.

In general, in FIG. 21(a), if x represents the black level of an inputsignal, as x becomes larger, the amount of gamma correction becomessmaller, while as x becomes smaller, the amount of gamma correctionbecomes larger. To determine the values L1, L2 and G2 and the value ofthe gain K of the amplifier 803 with respect to a specific x, thefollowing equations are employed:

    K=g1*(x4-x)/(x4-x2)

    L1=x

    L2=-f(x)

    G2=(f(x4)-f(x2))/(f(x4)-f(x))*g2

where the function f(x) represents the input-output characteristic ofthe gamma correction circuit 809, x2 represents the black level of theinput signal which is in a standard state, x4 represents the 100% whitelevel of the input signal which is in the standard state, and g2represents the value of G2. It is assumed here that the gain of theamplifier 803 at this time is set to K1.

Accordingly, to vary the gamma correction characteristic continuously inthe above-described manner, it is only necessary to vary the values ofK, L1, L2 and G2 while using x as a parameter in accordance with theabove equations.

FIG. 21(b) shows the gamma correction characteristics obtained throughthe above-described overall processing performed in the entireapparatus. The horizontal axis represents the output of the CCD 801,while the vertical axis represents the output of the D/A converter 815.In FIG. 21(b), the curve F indicates the gamma correction characteristicwhich provides the smaller amount of correction than in the normalstate, the curve G indicates the gamma correction characteristic whichis in the normal state, and the curve H indicates the gamma correctioncharacteristic which provides the larger amount of correction than inthe normal state. x6 indicates the output value of the CCD 801 at the100% white level, and x7 indicates the output value of the CCD 801 atthe saturation level thereof. As described above, in the part between x6and x7, each of the characteristic curves takes the form of the kneecharacteristic. As can be seen from FIG. 21(b), the gamma characteristicin the range between the black level and the 100% white level can bevaried by varying the values of G1, L1, L2 and G2, whereas the kneecharacteristic part in the range above the 100% white level does notsubstantially vary even if the gamma characteristic is varied.

In the output range of the gamma correction circuit 809, an actuallyusable range may occasionally become narrow as shown in FIG. 21(a). Thischaracteristic degradation due to a cancellation of significant bitsoccurring during gamma correction can be suppressed by preparing thegamma correction circuit 809 which is arranged to provide an outputhaving a resolution of (n+a) bits ("n" represents the number of unitbits which are used for conversion performed by the D/A converter 815and "a" is approximately 1-3). As can be seen from FIG. 21(a), if asignal of level lower than a black level enters the gamma correctioncircuit 809 owing to noise or the like, a signal whose level is lowerthan the black level may be outputted from the apparatus. In theopposite case, an excessively large output may be produced. The limiter814 is provided for preventing occurrence of such a problem.

The multiplier 812 and the memory 813 can also be used for otherpurposes. For example, they can be used for adjusting a signal level oreffecting the fade function of varying a signal with time.

The amplifier 803 can also be used in combination with an automatic gaincontrol amplifier arranged to raise the gain of the amplifier 803 when asubject of low illuminance is photographed. In this case, it is alsopossible to adopt an arrangement in which the gain of the amplifier 803can be varied by inputting the output signal of the memory 804 to areference voltage terminal of the automatic gain control amplifier.

The above-described embodiment is arranged so that its non-linearcorrection characteristic can be set to either a correctioncharacteristic which provides a larger amount of correction than anormal-state correction characteristic or a correction characteristicwhich provides a smaller amount of correction than the normal-statecorrection characteristic. However, the present invention is not limitedto this arrangement, and it is also possible to adopt an arrangement inwhich the non-linear correction characteristic can be set to either oneof the former and latter correction characteristics. Although the abovedescription has referred to the non-linear circuit having the gamma andknee characteristics, the present invention can of course be applied toa non-linear circuit having a non-linear correction characteristic otherthan the gamma and knee characteristics.

As is apparent from the foregoing description, in accordance with theembodiments of the present invention, since it is possible to realize anon-linear circuit without using a ROM, it is possible to implement anon-linear circuit having a reduced circuit scale. Accordingly₁ it ispossible to achieve highly effectively a cost reduction, a reduction inthe power consumption of the apparatus and a reduction in the entiresize of the apparatus.

Also, since the value and inclination of each function coincide withthose of the adjacent function at each function switching point, it ispossible to realize the advantage of preventing image degradation fromoccurring due to the switching of the functions.

Further, since the number of bits at the input of the multiplier may besmaller than the number of bits of an input signal, the circuit scalecan be greatly reduced.

Also, since it is possible to form the circuit as an integrated circuitof reduced chip size, a reduction in manufacturing cost can be achieved.Also, since a non-linear characteristic, such as a gamma characteristicor a knee characteristic, can be finely varied according to theconditions of a subject or the state of photography it is possible toperform photography in an optimum state at all times. Further, since agamma characteristic and a knee characteristic can be realized by asingle circuit, the required circuit scale can be reduced.

In accordance with the above-described embodiments of the presentinvention, it is possible to vary a non-linear correction characteristicby means of a simple arrangement, so that the non-linear correctioncharacteristic can be varied according to the state of photography orthe condition of a subject to be photographed. Among others, it ispossible to vary the non-linear correction characteristic over a widerange by varying constants for use in addition and subtraction withoutvarying a coefficient term, and, in addition, no ROM table is needed andonly one multiplier having a large circuit scale is used. Accordingly itis possible to achieve a large variation effect without an increase incircuit scale.

Further, in accordance with the above-described embodiments of thepresent invention, since a gamma characteristic can be varied withoutsubstantially varying a knee characteristic, no dynamic range isimpaired.

What is claimed is:
 1. An image pickup apparatus arranged to convert apicked-up image signal from analog to digital and perform signalprocessing in a digital manner, comprising a gamma circuit having agamma region in which a gamma characteristic is obtained by addingtogether a straight line passing through an origin and a first curve anda knee region in which a knee characteristic is obtained by addingtogether a predetermined value and a second curve.
 2. An image pickupapparatus according to claim 1, wherein the first curve is obtained bymultiplying a third curve by a first coefficient and the second curve isobtained by multiplying a fourth curve by a second coefficient, each ofthe first coefficient and the second coefficient being variable.
 3. Animage pickup apparatus according to claim 2, wherein the third curve isformed by not less than one quadratic function and the fourth curve is astraight line, the third curve and the fourth curve being each obtainedby varying a coefficient of one function circuit including a multiplierin accordance with an input level.